The invention relates to flow control devices. It relates, more particularly, to flow control devices in which a valve is operated by servo means responsive to a mass flow signal derived from a flow sensor.
The prior art in the field of fluid flow control is extensive and relates to both flow sensors -- devices for measuring the rate of fluid flow in a channel -- and control valves -- devices for altering the rate of flow in a given channel by varying the resistance in the path of such flow. The prior art has also taught the construction of control circuits capable of operating control valves in response to signals derived from flow sensors and compared to signals representing the desired rate of flow.
One common difficulty with control valves of the prior art relates to their incorporation of seals preventing the agress of the fluid from the internal volumes of the valve to the atmosphere. Such seals are commonly associated with the valve operators which penetrate the valve body.
The novel control valve disclosed herein uses no sliding or rotating seals, and the operating member does not penetrate into spaces occupied by the controlled fluid. The flow coefficient of the valve is varied by compressing a ball against a core assembly consisting of a length of elastomeric tubing and a solid anvil. Friction is minimized in the control valve of the invention and the torque required to operate is reduced considerably, with respect to designs of the prior art.
The invention also encompasses a flow sensor of improved design. The prior art has taught the use of a self-heating thermo-electric element, located at some selected position in the flow stream, for the measurement of mass flow inside a conduit, and compensation for temperature variations by the use of additional thermoelectric elements located at some other selected positions inside the conduit. In the flow sensor of the invention the accuracy, reproducibility and sensitivity of the device has been improved significantly by the following features: (a) measurement of the mass flow rate is facilitated by interacting a self-heating thermo-electric sensor with a hydrodynamically focused fluid stream, and (b) compensation for changes in the ambient temperature is facilitated by using two passive thermal elements which may be located anywhere in the flow conduit, except in the immediate vacinity of the focused stream.
It is, therefore, a primary object of the invention to provide a flow control valve in which the actuating elements are sealed from the fluid within the valve by stationary seals not exposed to motion, either reciprocating or rotary.
It is a further object of the invention to provide a novel flow sensor, and associated electrical circuitry, which minimizes influences which would degrade the performance of the sensor, and which permit ready and simple compensation for temperature variations in the fluid stream.
It is yet another object of the invention to provide a flow control device integrating a novel flow sensor and a novel flow control valve with a control circuit particularly adapted to both.